Unidirectional Air-Surface Exchange

The flux of atmospheric pollutants to Earth’s surface follows the same air-surface exchange mechanisms as the surface fluxes of heat, moisture and momentum and are also influenced by the physical, chemical, and biological properties of the surface (Hicks et al, 1987). For chemical species with no sources at the land or water surface, the flux can be parameterized in air-quality models as the product of the ambient atmospheric concentration and the dry deposition velocity.

In CMAQ, the unidirectional dry deposition velocity is parameterized using an analogy with Ohm’s Law where the transfer along pathways is modeled as a set of resistances that act in series and parallel. Improving the characterization of these resistances in CMAQ requires a better understanding of the underlying processes. EPA is conducting specialized field studies which will help to improve these parameterizations of deposition velocity and therefore reduce uncertainties in flux estimates.

In addition to improving the model algorithms, improved input data will also be developed for use in modeling deposition. More resolved land use/land cover data will be integrated in CMAQ as well as improved surface characterizations such as LAI and canopy height.

The improved CMAQ deposition model will be linked to other media models and will be used in studies within the linked system to develop data layers for input to the National Atlas and for use in studies such as critical loads. In this way, the improved air-surface exchange processes and land cover characterization in CMAQ can be used to predict the connection between air concentrations and deposition for development, implementation and review of NAAQS, for critical loads, and for improved linkage to ecosystem models.